Cadmium telluride is a photoactive material which has revealed itself to be particularly suitable for manufacturing solar cells, since it is characterized by a so-called “energy gap” with a value of 1.45 eV which is highly favorable. Moreover, polycrystalline layers of CdTe with a thickness of a few micrometers (μm) can be deposited easily by way of several technologies, including Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD), Close-Space Sublimation (CSS), screen-printing, spray methods and others. However, such technologies require generally a certain type of treatment after deposition in order to optimize the properties of the active layers of the cell.
CdTe was used for the first time in the manufacture of solar cells in the 1960s and, at the end of the 1970s, the efficiency of such cells had reached a value of 9%. The production process used entails doping the CdTe by introducing oxygen in the material deposition step, working at high temperatures (560-580° C.). A post-deposition treatment in an oxygen atmosphere also contributes to a further 1-2% increase in terms of efficiency. However, this process has revealed itself to be scarcely practical, since it is scarcely controllable and expensive, especially due to the high costs of the substrates made of heat-resistant glass required for the procedure.
Subsequently, in the 1980s, a method for CdTe deposition of the Closed Space Vapor Transport (CSVT) type was developed. This process utilizes the favorable properties of the heat-resistant glass support, which is heated rapidly to the temperature of 650° C. The CdTe is then deposited by the source heated at 600° C.
Deposition methods of the electrolytic type were also developed in the 1980s. In this case, an aqueous solution of CdSO4 and Te2O3 at the temperature of 90° C. is used. However, such electrolytic methods require long times for deposition, since the deposition rate must be kept low in order to avoid the development of fluctuations in the stoichiometry of the CdTe layer.
Screen-printing technology instead uses a suspension of particles of Cd and Te dust, which is deposited on the support and then converted into a relatively thick CdTe layer by means of a thermal treatment at high temperatures (above 700° C.). In this case also, the method is expensive due to the cost of the suitable substrates.
Deposition methods of the spray type utilize an aqueous solution of components which contain Cd and Te, which is atomized and deposited in the form of droplets onto the support, which is heated to 400° C. Since the CdTe layer that is deposited tends to form porous structures, the deposition must lead to the formation of a thick layer, which can help prevent the subsequent permeation, within the cavities of the CdTe layer, of the material that constitutes the layers to be deposited later. This aspect, combined with the great quantity of material that is wasted during the atomization step, causes spray methods to be relatively onerous.
There is, therefore, the need to develop a process for preparing a solar cell based on CdS and CdTe in which low-cost precursors are used in quantities which are not excessive, in which relatively low temperature conditions are used, and which can be applied easily in the industrial sector to allow inexpensive mass-production of solar cells.